yum update fails with HTTP 503, but curl succeeds?

I'm new to CentOS... I've just done a basic installation of CentOS 7. The only thing I've touched so far is the network device to enable it. After confirming internet connectivity, I tried to update the system:

[root@hostname ~]# yum update
Loaded plugins: fastestmirror
Could not retrieve mirrorlist http://ift.tt/ZFTCSa error was
14: HTTP Error 503 - Service Unavailable


 One of the configured repositories failed (Unknown),
 and yum doesn't have enough cached data to continue. At this point the only
 safe thing yum can do is fail. There are a few ways to work "fix" this:

     1. Contact the upstream for the repository and get them to fix the problem.

     2. Reconfigure the baseurl/etc. for the repository, to point to a working
        upstream. This is most often useful if you are using a newer
        distribution release than is supported by the repository (and the
        packages for the previous distribution release still work).

     3. Disable the repository, so yum won't use it by default. Yum will then
        just ignore the repository until you permanently enable it again or use
        --enablerepo for temporary usage:

            yum-config-manager --disable <repoid>

     4. Configure the failing repository to be skipped, if it is unavailable.
        Note that yum will try to contact the repo. when it runs most commands,
        so will have to try and fail each time (and thus. yum will be be much
        slower). If it is a very temporary problem though, this is often a nice
        compromise:

            yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true

Cannot find a valid baseurl for repo: base/7/x86_64

However:

[root@hostname ~]# curl "http://ift.tt/ZFTCSa"
http://ift.tt/1E1U92B
http://ift.tt/1CANFUv
http://ift.tt/1E1U92F
http://ift.tt/1E1U92H
http://ift.tt/1E1Ubru
http://ift.tt/1E1UbHK
http://ift.tt/1E1UbHM
http://ift.tt/1E1UbHS
http://ift.tt/1CANGaT
http://ift.tt/1E1U92L

/var/log/yum.log is empty. Here is /etc/yum.conf:

[main]
cachedir=/var/cache/yum/$basearch/$releasever
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
installonly_limit=5
bugtracker_url=http://ift.tt/1CANGaX
distroverpkg=centos-release


#  This is the default, if you make this bigger yum won't see if the metadata
# is newer on the remote and so you'll "gain" the bandwidth of not having to
# download the new metadata and "pay" for it by yum not having correct
# information.
#  It is esp. important, to have correct metadata, for distributions like
# Fedora which don't keep old packages around. If you don't like this checking
# interupting your command line usage, it's much better to have something
# manually check the metadata once an hour (yum-updatesd will do this).
# metadata_expire=90m

# PUT YOUR REPOS HERE OR IN separate files named file.repo
# in /etc/yum.repos.d

There is no proxy server in this network.